This work was undertaken to clarify the criteria for classifying pyrethroids into Types I and II on the basis of their structure-activity pattern. Thus, effects on the membrane potential in crayfish giant axons of substituted benzyl esters of chrysanthemic, α-substituted iso-valeric, and related carboxylic acids with and without the cyano group at the α-position of the alcohol moiety were examined by an intracellular recording technique. Some α-cyano-benzyl alcohol esters caused a remarkable depolarizing afterpotential in response to a single stimulus similar to that induced by esters without the cyano group. Their effects on sodium currents were examined with an internally perfused crayfish axon under voltage clamp conditions using the sucrose gap method. Irrespective of the presence of the α-cyano group in the alcohol moiety, these compounds induced a tail current upon a step repolarization of the membrane, although the α-cyano compounds decreased the rate of fall of the tail current much more slowly than that of the corresponding compounds without the cyano group. Insecticidal activity of compounds was measured against the American cockroaches under synergistic conditions with metabolic inhibitors. The activity of compounds with the α-cyano group was much higher than that of the corresponding compounds without the cyano group. The higher the tail current activity, the higher was the insecticidal activity, although contributions of such structural effects as hydrophobicity may differ between the two activities. The results suggest that the presence of the cyano group at the α-position of the benzyl alcohol moiety of synthetic pyrethroids enhances both the sodium current and insecticidal activities but is by no means a sufficient structural requirement to differentiate the types of each of the biological activities.